It isn't as easy as swapping engines. Instead you'll have a whole list of upgrades that you must suddenly tackle. I don't know much about automatics but I do know the process if you have a manual. Some of the following will certainly apply if you have an automatic, however.
For one, you'll need another bell housing, clutch, shift fork, and maybe retainer bearing. The shift fork is longer with the 9.5" bell housing, so when you go to a 10.5" clutch and bell housing you need a short style later shift fork. So say to hello to much harder clutch pushing with your larger clutch and decreased mechanical advantage.
Connected to that drive line is your small stock 7-1/4" rear end, which is too small for the added power. Now you need an 8-1/4" or 8-3/4" or maybe even a Dana or Ford rear end, which probably needs to be shortened (cut and welded) to fit your Barracuda .
Obviously you'll need a new drive line, especially since you'll likely go with a newer more "standard" yoke-style drive line.
Next, you'll likely need to upgrade or at the very least adjust your torsion bars. A larger engine like a 340 or 360 with larger bell housing will sag your front end. The Commando had bigger torsion bars for a reason.
You'll probably need to rebuild your original radiator to a three core design and maybe add a clutch fan while you're in there to accommodate the higher engine temps.
Of course, none of this matters if you're still stuck using the tiny/crappy/dangerous stock drum brakes. So you can bump them to larger drums or go with a disc brake setup. I have Scarebirds up front and 10" drums in the back (although I recently bought an SSBC rear disc kit for the back).
If you go with larger brakes, and in my opinion you absolutely must, you need 14" rims or better.
Oh, guess what? The best fitting exhaust manifolds for any small block are the stock 273 manifolds, which are adequate for the 273 but are quite restrictive for any other small block. Also, when fitting them you'll notice that the ports are offset even further by the later manifold port design on the heads - so now they're even more restrictive. So the manifolds will choke a later V8. You can try to find the '70-'72 Hi-Po manifolds, but they're expensive and really don't make a much greater opening and will absolutely NOT work with power steering. So now you're stuck with expensive headers that prevent anything else from fitting, are a ***** to install, and necessitate either a custom hydraulic clutch or re-welded Z-Bar that, if it fits, will barely fit at all.
Last, you'll need a new mini-starter because that big old honking stock starter won't fit any more.
So yes - technically you can bolt in another small block but be aware that it's very ignorant advice if someone claims that you can just pop in any old 360 small block and be on your way. If you do be prepared for the above mods otherwise your experience will range somewhere between inadequate to dangerous.
I know all of this from direct experience on my own early As. When I first got into these cars many years ago a bunch of Mopar guys claimed up and down that you could throw a 360 in there and go for it. I listened to them and found them wrong. I have a 360 and you can too, but it's not as easy as a Saturday afternoon engine swap.
Just to sound like the old man that I am, think about what the car is and what your goals are. I like to think of the Early As as less of a hotrod and more of a gentleman's cruiser. If you want to take it easy on yourself, hop up the 273 to Commando specs or go with a 318 which is basically a bored out 273. If your ***** doesn't need the huge horsepower boost, then having your '65 with a 273 or 318 with reasonable power, reliability, and safety upgrades will keep you smiling wide more often. It all depends on the user. Some guys like to work on their cars all of the time, others like to drive them.
Sorry for the length. I'm an old guy and just wanted to keep you from getting bad advice.
-Red
